As a projection display apparatus which displays a projected image, for example, known are a liquid crystal projection display apparatus (for example, see JP-A-2001-264728) in which, as shown in FIG. 8, incident light is modulated in accordance with a video signal and transmitted light is projected to display an image, and a DMD (Digital Micromirror Device, a registered trademark) type projection display apparatus (for example, see JP-A-2004-361856) in which, as shown in FIG. 9, the reflection angles of micromirrors disposed for respective pixels are changed in accordance with a video signal to modulate the quantity of reflected light, and light that is obtained as a result of the modulation is projected to display an image.
FIG. 8 is a diagram showing the basic configuration of a conventional liquid crystal projection display apparatus 80. Light emitted from a light source 1 is reflected by a reflecting mirror 2 to be formed as light which is substantially parallel to the optical axis. The parallel light enters a polarizer 4 through an illumination optical system 3, and is converted by the polarizer 4 into light of a predetermined polarization direction. The light emitted from the polarizer 4 is subjected to modulation of the polarization state by a liquid crystal panel 5 which is an image-displaying spatial light modulator, and then incident on an analyzer 6. When the light is passed through the analyzer 6, only a predetermined polarization component is emitted to a projection lens system 7, and passed through the projection lens system 7 to be projected and displayed onto a screen or the like which is not shown.
FIG. 9 is a diagram showing the basic configuration of a conventional DMD (digital micro mirror device) projection display apparatus 90. Light emitted from a light source 1 is reflected by a reflecting mirror 2 to be formed as light which is substantially parallel to the optical axis. The parallel light enters a DMD (digital micro mirror device) device 905 which is an image-displaying spatial light modulator, through an illumination optical system 3. The light which is reflected by micromirrors of the DMD (digital micro mirror device) device 905 to undergo the light quantity modulation enters a projection lens system 907, and passed through the projection lens system 907 to be projected and displayed on a screen or the like which is not shown.
It is pointed out that a projection display apparatus is lower in contrast than an image display apparatus which is not the projection type, such as a CRT. The contrast is a ratio of a luminance obtained when a white screen is displayed, and that obtained when a black screen is displayed. In a projection display apparatus, even when a black screen is displayed, stray light is incident on a projection lens, and therefore the contrast is lowered.
In the liquid crystal projection display apparatus 80, recently, a variable optical shutter or an aperture is disposed in the illumination optical system 3 or the projection lens system 7 in order to improve the contrast (for example, see JP-A-2001-264728). The disposition of a variable optical shutter or an aperture can reduce, for example, light that has a large incidence angle to the liquid crystal panel 5 which is an image-displaying spatial light modulator, or a large emission angle from the liquid crystal panel 5.
By contrast, in the DMD (digital micro mirror device) projection display apparatus 90, a black screen is displayed by swinging the micromirrors so that the light reflected from the DMD digital micro mirror device) device 905 which is an image-displaying spatial light modulator is not incident on the projection lens system 907. However, the DMD (digital micro mirror device) device 905 is an aggregation of the micromirrors, and hence scattered light is caused in peripheral portions of the micromirrors and among the micromirrors, so that part of scattered light is incident on the projection lens system 907. In the DMD (digital micro mirror device) projection display apparatus 90, the contrast can be enhanced by disposing an aperture in the projection lens system 907.
On the other hand, there is a further problem in that, when an aperture is disposed in a conventional projection display apparatus, the luminance of a white screen to be displayed is lowered. As a countermeasure against the problem, a technique is disclosed in which an aperture is configured so that the area of an opening (hereinafter, referred to as an opening amount) or the quantity of blocked light can be changed to a predetermined level (for example, see Patent Reference 2). Hereinafter, such an aperture is referred to as a variable aperture. The problem of the contrast of a projected image arises depending on the brightness of an environment where the projected image is displayed.
In a bright room which is illuminated with external light such as illumination light or sunlight, a screen is illuminated by the light regardless of whether a projected image is displayed or not. Even when a black display portion is slightly bright, therefore, there arises no problem, and a white screen is required to have luminance which can cancel the external light. By contrast, in a dark room which is not illuminated with external light, high luminance is not required, and a black screen having even a slight brightness is conspicuous. A variable aperture is adjusted so that, in an environment where external light exists, a white color is brighter, or the opening is widened so as to attain high luminance, and, in an environment where external light does not exist, the opening is narrowed so as to suppress a white color and enhance the contrast. Moreover, the opening amount is adjusted also in accordance with a projected image to be displayed.
In a conventional projection display apparatus having a variable aperture, however, the opening amount is adjusted by a mechanical system using a motor and the like, and hence there is a problem in that vibrations, noises, and the like are generated in the adjustment of the opening amount. Such vibrations, noises, and the like obstruct watching movies. There is also a variable aperture having no mechanical system, such as a variable aperture using TN liquid crystal, or that using polymer-dispersed liquid crystal. In a variable aperture using TN liquid crystal, the polarization direction of light which is incident on TN liquid crystal must be restricted to a specific polarization direction. Therefore, a polarizing plate is required. When a polarizing plate is used, however, there arises a problem in that the use efficiency of light is reduced. The reduction of the use efficiency of light is caused by the fact that the transmittance of a polarizing plate which is used for restricting the polarization direction of light that is emitted from a light source, and that is randomly polarized, to linear polarization of a specific polarization direction is 50% or less, or the fact that, even when the polarization direction is converted by using a polarization beam splitter, the polarization efficiency is about 70%. When light is to be blocked, light which has been transmitted through TN liquid crystal must be absorbed by a polarizing plate, and hence there is a problem in that the polarizing plate is heated by the light absorption, and the reliability is lowered. In a variable aperture using polymer-dispersed liquid crystal, the quantity of transmitted light is reduced by scattering incident light, and the scattering angle is small. Also when light is to be blocked, therefore, part of scattered light reaches a screen, thereby causing a problem in that light cannot be completely blocked.